We have observed that tissue lymphocytes, presumably in various stages of differentiation and levels of activity, can differ considerably with respect to the amount of spectrin associated with a previously undescribed cytoplasmic organelle that may function as a spectrin organizing center. We have recently linked spectrin repositioning within the cell and this organelle to T cell receptor mediated activation. This conclusion was based upon our observation that the spectrin within this organelle rapidly fragments upon cell activation and individual 'packets' of spectrin are translocated to the plasma membrane. Based upon this new observation linking the cytoskeleton to lymphocyte function significance of this newly characterized spectrin-rich organelle, and more generally help us to determine the physiological significance of the natural heterogeneity in spectrin organization and its distribution seen among tissue lymphocytes. We will 1) determine the role of activation signals such as Ca2+, calmodulin and protein kinase C activity in the regulation of the spectrin aggregate and its fragmentation and translocation, 2) examine the consequences upon activation of inhibiting spectrin expression or blocking its function, 3) conduct an extensive structural evaluation of the aggregate including its solubility properties, its protein composition and relationship to other cytoskeletal components, 4) determine the spatial relationship of the positioning of the spectrin aggregate (and its subsequent movement to the plasma membrane) and the site ont he membrane at which a signalling event takes place, and 5) to study peripheral lymphoid tissues and isolated lymphocytes, and the thymus during development (i.e., correlation with T cell receptor expression) to determine the functional significance of the heterogeneity in spectrin distribution observed in situ. These studies form the basis by which a new cellular response to activation will be characterized and will also contribute toward an understanding of how structural differences among lymphocyte subsets contribute to their activation potential, memory, and the specialized function of each subset.